In addition to some measure of conventional shear strength, a desirable feature for testing devices for materials for flexible pavements, is to be able to indicate the optimum bitumen content or water content along with the optimum unit weight (density), as well as the shear strength and the dynamic moduli of the compacted test specimen when subjected to cyclic loading. The optimum bitumen content for current paving materials design tests, employing a fixed compaction effort, are largely based upon empirical correlations with limited ranges of voids, i.e. empirical voids criteria. This approach is not universally applicable because the void content is a variable and different criteria are needed when the material changes or the compaction effort changes. Some paving materials design tests employ tamping as the compaction method where the number of tamps of a given magnitude (impact of kneading) are empirically correlated with the unit weight (density) developed under traffic, and these become obsolete when the traffic loads change.
Other machines for testing paving materials by kneading under pressure are known. In U.S. Pat. Nos. 2,972,249 and 3,478,572 shear strain and unit weight (density) are tested by a kneader compactor which is essentially a mechanical analog of the pavement structure which permits adjusting the design stress in accord with the anticipated pavement design loading. In U.S. Pat. No. 3,478,572, a device for measuring wall friction of the sample with respect to the mold is also provided.
In machines of this type, a mold chuck receives a cylindrical mold containing a sample of material such as gravel, stone or soil, usually mixed with bitumen or water. The sample is subjected to a controlled compressive load by a hydraulic ram and to controlled shear strain by a oscillator mechanism which applies a gyratory motion to the chuck into which the mold is clamped. The gyratory angle response is a measure of shear strain under the applied loading and is thus an indication of the plastic properties of the sample. As the sample is kneaded and compacted the gyration varies in response to changes in the physical properties of the sample and the point at which it starts to progressively increase with continue kneading is significant in that it indicates a critical degree of saturation (percentage of voids filled) in the densification process.
Other machines used previously for measuring the dynamic moduli (compression and rebound) under cyclic loading have simply employed cyclic loading on a confined or unconfined cylinder of the paving material--the cylinder receiving no added distortional stress in conjunction with the cyclic vertical stress. This is not analogous to the action occurring in the pavement layer beneath a moving wheel as the pavement layer actually deflects under the load thus introducing a combined action of internal particle movement or so-called kneading action in conjunction with the vertical compression under the wheel load and rebound with the passage of the wheel load.
The prior art does not provide a machine that will measure the required design parameters and furnish information displays that permit automatic, convenient and positive determination of the relevant parameter of optimum bitumen content or water content along with the relevant unit weight (density) and dynamic moduli and strength properties under dynamic conditions for use in designing flexible type pavement.